The present invention relates to a magnet device for generating a static magnetic field in magnetic resonance imaging (MRI), and more particularly to the magnet device, provided with a main coil assembly and a sub-coil assembly for shimming the static magnetic field created by the main coil assembly, which generates a horizontal magnetic field in which magnetic lines of force pass along a patient's body axis.
In magnetic resonance imaging, there is provided a magnet device for generating a static magnetic field, whereby atomic nuclei of a patient body align themselves with the static field. For high resolution and non-distortion of images produced, a static magnetic field is needed, as one of the requirements for MRI, so as to have a highly uniform magnetic field providing a practical diagnostic space. The diagnostic space having high uniformity of magnetic flux can be accomplished by adopting shim coils.
U.S. Pat. No.4,506,247 discloses even and odd pairs of axisymmetric correction coils (i.e. a shim coil assembly) disposed on a cylindrical coil form at specified longitudinal positions without overlap. The purpose of these coil pairs is to provide a correction field for a magnet employed in MRI systems. The correction is given by selecting properly the locations of the coils on the cylindrical coil form which is disposed coaxially with a main magnet device such as a permanent magnet device or a superconducting magnet device.
However, applying the above correction coil system to a main magnet device will result in an increase of the size in the radial direction of the device, because the cylindrical coil form must be installed, separately from and coaxially with the main magnet. Moreover, assembling of the whole magnet device would be cumbersome. Further, two power units for the main coils and correction coils are needed.
Japanese Patent Laid open No. 62-293704 also discloses a shimming mechanism for a vertical magnetic field in which magnetic lines of force pass through a patient's body from his front to his back or vice-versa. The above shimming mechanism adopts at least one pair of ring-like sub-coils, which is disposed, together with main coils producing a main magnetic field, within a fixed angle range with respect to the central point of the static magnetic field. The sub-coils are to receive electric current permitting them to generate a magnetic field opposite to the main magnetic field in order to acquire a uniformity of the diagnostic space.
Even in the magnet device generating the horizontal magnetic field, it is desirable to create a large diagnostic space which necessarily leads to a far distance between the coils of a main coil pair. Since, when the device of generating the horizontal magnetic field utilizes the above shimming mechanism, the device will be increased in size in the axial direction, because the sub-coils must exist in the main coil assembly. Besides, accommodating the main coils and sub-coils within the predetermined angle region with respect to the central point will result in a bulky size in its radial direction.
It is also required for the magnet device to provide a large diagnostic space having a high strength uniform magnetic field. A patient may be troubled with claustrophobia due to a small, tight diagnostic space during the examination. To avoid such discomfort, the size of the magnet device will have to be increased in both the axial and radial directions, which is undesirable. Especially, in a superconducting magnet device which is desirable for generating a high strength of magnetism, a high-strength, high-uniform spherical diagnostic space of 30 to 50 cm diameter would lead to a noticeably elongated axial form for keeping a less-error axial region as long as possible.